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Video transcript

what you see depicted here is known as a pedigree and a pedigree is a way of analyzing the inheritance patterns of a trait within a family and it can be useful to understand more about that trait maybe to make some insights about the genetics of that trait and it's a way to think about well what's happened in the past in a family and then maybe we can help get some probabilities or get some understanding of what might happen in the future but let's just break it down so that we understand all of the different parts of a pedigree so a big picture a circle represents a female in the family and a square represents a male in the family if either the circle or the square is colored in like we see right over here or we see right over here that means that that person exhibits the trait and the trait that this particular pedigree is for is the trait of freckles and whether or not we're colored in that is a phenotype that is an observed trait you just look at the person you say do they have freckles or not this male right over here does not have freckles this female right over here does have freckles this male right over here does have freckles but once again this phenotype that's the observed characteristic we don't know exactly the genotype just from this although we might be able to infer some ideas about the genotype by seeing how the phenotype pattern spread from generation to generation but just going back to understanding the pedigree itself when you have a horizontal line connecting a male and a female it's called a marriage line and it means that they are coupling up and they are reproducing they don't necessarily I guess have to be married it's more that they reproduce so this male and this female have reproduced and then this vertical line that goes from that horizontal line that's known as the line of descent so it's going from that first generation to the second generation and so all of the people connected to that vertical line at the bottom of the vertical line these are their children all of the people who are directly connected to this and this is known as a sibling line so this person this person and that person they are all brothers and sisters you can see that generation 1 they had two daughters one of whom exhibited freckles and one son who had freckles now these other people in generation 2 these aren't children of the first generation these are people you could say who are brought into the family they either married into the family or you could just say they reproduced with the children but that's what's happening with generation 2 and so you can see you have another what's known as a marriage line but I guess it could be a coupling line a line of descent and then they have two daughters in this case so it turns out that freckles is an autosomal dominant trait and I will explain what that means in a second autosomal dominant dominant trait autosomal means that it is a trait associated with one of the non-sex chromosomes we have 23 pairs of chromosomes as humans one of those pairs determines our sex the other ones don't determine our sex so this means that freckles is associated with one of the 22 pairs of chromosomes that do not determine your sex and dominant means that in general you get two versions of a gene one from your mom and one from your father and each of those versions of the gene for say whether or not you have freckles is known as an allele and if a trait is autosomal dominant that means that if you have even just one allele for that trait that you are going to exhibit the trait and so for example if the alleles for our freckles if capital F is you have freckles its dominant so we write it in capital and lowercase F means that you don't have freckles if someone has a genotype of capital F capital F or capital F lowercase F or I guess we could say lowercase F capital F if we wanted to think about order which one you're getting from which parent well then in all of these situations they would exhibit the phenotype of freckles and we talked about genotype and phenotype in other videos and of course the only genotype in which they would not exhibit freckles would be having both of the recessive traits so with this out of the way what can we infer about the genotype of this individual right over here and this individual will sometimes be referred to as individual one two there in generation one and they are individual two in generation one well we know that they exhibit freckles so we know for sure that since the phenotype is freckles they are in one of these scenarios and these two these two are frankly equivalent so this person we know that they're either capital F capital F so that would be homozygous dominant or they're capital F lowercase F which are they they are heterozygous well let's first think about the situation if they were homozygous dominant if they're are homozygous dominant that means to every one of her children she would give one of these alleles and either of those are dominant and so that would mean if this were the case that all of her children would get at least one dominant allele well if they all get at least one dominant allele then all of her children would have freckles but we're seeing a situation where all of her four children do not have freckles two of her three children had freckles so this cannot be the case so this individual right over here must be heterozygous for freckles so they must be capital F lowercase F and what do we know about this individual right over here the person that she coupled up with well they don't exhibit the phenotype for freckles so they must be lower case F lower case F and then from that you might be able to intuit a lot more about that and we'll do that in future it worked examples
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